1. Field of the Invention:
This invention relates to zoom lenses and, more particularly, to zoom lenses using an aspheric surface in the lens system at an appropriate location to get as large a relative aperture as 1.7 in F-number for the wide-angle end with as high a zoom ratio as 10 to 25, while still maintaining good optical performance throughout the entire zooming range, and suited to television cameras, photographic cameras, or video cameras.
2. Description of the Related Art:
In the field of art of television cameras, photographic cameras, or video cameras, there has been a growing demand for zoom lenses of large relative aperture and high range with good optical performance.
Of these, especially the color television camera for broadcasting has to be quick and easy in handling and good in manageability, which are regarded as important. To meet such requirements, even the image pickup devices in recent years are taking a smaller size of CCD (solid-state image sensor) such as 2/3, or 1/2 inch ones in the main stream.
This CCD has its resolution almost uniform over the entire area of the image frame. To ensure that a zoom lens is usable with this CCD, therefore, the resolving power of the zoom lens must be uniform from the central to the marginal zone of the full aperture.
For this purpose, of all aberrations, for example, astigmatism, distortion and lateral chromatic aberration are necessarily subjected to good correction for high optical performance over the entire area of the image frame. It is further desired that the zoom ratio is high, while the size and the weight are nonetheless small and light. In addition, the back focal distance is so long that a color separating system and various filters can be put in front of the image pickup device.
The zoom lens of the type comprising, from front to rear, a focusing or first lens unit of positive refractive power, a second lens unit of negative power for varying the focal length, a third lens unit of positive or negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power for forming an image, or the so-called 4-unit zoom lens, has found its use in many color television cameras for broadcasting, since it is comparatively easy to increase the zoom ratio and the relative aperture.
Of the 4-unit zoom lenses, a large relative aperture, high range one of 1.6 to 1.8 in F-number whose range is 20 is proposed in, for example, Japanese Laid-Open Patent Applications No. Sho 54-127322 and No. Sho 51-14034.
Given a zoom lens of large relative aperture (F-number: 1.6-1.8) and high range (zoom ratio: 10-40), to include high optical performance over the entire zooming range, there is need to set forth proper design rules for the refractive power of each lens unit and for the construction and arrangement of the constituent lenses.
In general, to maintain good stability of aberration correction for the high optical performance throughout the entire zooming range, it becomes necessary to increase the degree of freedom on the aberration correction by, for example, increasing the number of lens elements in each lens unit.
For this reason, if it is attempted to get the large relative aperture, high range zoom lens, the total number of constituent lenses would unavoidably increase and a problem would arise in that the size of the entire system comes to increase largely.
In respect to the imaging performance, as one sees the central zone of the full aperture which gives best contrast to the image, or the so-called best image focus, variation of it with zooming comes to be a problem. This is ascribable mainly to the variation of spherical aberration with zooming.
The variation with zooming of spherical aberration in general is shown in FIG. 17, where letting the zoom ratio be denoted by Z and the shortest focal length by fw, the spherical aberration increases in the minus direction, or, its under-correction for the Gauss image plane grows, as zooming goes from the wide-angle end, where the spherical aberration is zero, to a position for a focal length fw'=fw.times.Z.sup.1/4. After having left that zooming position, the amount of under-correction starts to decrease, reaching zero at a certain zooming position. From this on, the spherical aberration now changes to over-correction.
The over-correction increases to a maximum at or near a zooming position for a focal length fd=ft.times.(F.sub.NO.w /F.sub.NO.t), where ft is the longest focal length and F.sub.NO.w and F.sub.NO.t are the F-numbers at the wide-angle and telephoto ends, respectively. It is at this zooming position that the F-number starts to increase (the lens system gets darker). In other words, the F drop begins from this position. As zooming goes on, the amount of over-correction decreases, reaching almost zero in the telephoto end.
Meanwhile, if the F-number in the telephoto end is greater by 5% or below than that in the wide-angle end, or, almost no F drop takes place, the zoom lens has its spherical aberration, as shown in FIG. 18, most over-corrected just before the telephoto end, or at a zooming position for a focal length ft'=fw.times.Z.sup.3/4. On going past this zooming position, the amount of over-correction decreases. In the telephoto end, it tends to change to slight under-correction.
So, if correction is otherwise made to bring the spherical aberration to almost zero in the telephoto end, the over-correction in the zooming position for the focal length ft' even grows greater.
In such a manner, for the zoom lenses, particularly the one having the start point of the F drop within the zooming range, the spherical aberration in the telephoto end is very difficult to control. In the prior art, therefore, it has been the common practice to increase the number of constituent lenses in either of the lens unit for focusing and the lens unit for varying the focal length in order to lessen the variation of spherical aberration with zooming. For this reason, there is a problem that the whole lens system increases in the bulk and size and becomes complicated in the structure of construction.